This invention relates to carboxy terminated polymers.
Normally solid carboxy-containing polymers of conjugated dienes alone or copolymerized with vinyl aromatic monomers have utility as modifiers in thermosetting polyester resin compositions as disclosed in U.S. Pat. No. 4,020,036. These carboxy-containing polymers can be made by variety of methods as disclosed in the above patent. One method for making such carboxy-containing polymers is based on solution polymerization techniques wherein the monomers such as 1,3-butadiene and styrene are polymerized in a solution with a hydrocarbyl lithium initiator under conditions such that a block copolymer of butadiene and styrene is formed. The polymerization mixture is then contacted with carbon dioxide to convert the polymerlithium to polymer-carboxy-lithium. The carbonated reaction mixture can be further treated under hydrolysis conditions to convert the carboxy-lithium end group to a carboxylic acid end group during polymer recovery operations.
While straightforward in theory, the carbonation procedure for preparing carboxy-containing polymers has certain disadvantages. First, the carbonation procedure requires a very high efficiency of mixing during the reaction of the polymerlithium in hydrocarbon solution with carbon dioxide in order to achieve a high degree of carboxy content in the final polymer. Thus, expensive, specially constructed mixing equipment is generally used for the carbonation step. Carbonation with CO.sub.2 can also be improved by using a large excess of CO.sub.2 (&gt;40 times stoichiometric amount) and low reaction temperature (&gt;25.degree. C.). But these conditions are not commercially attractive. Even under very good mixing conditions an appreciable portion of the polymer product does not contain a carboxyl group because of a competing coupling reaction which involves an intermediate polymer molecule containing a carboxy group or carboxylate salt group. While the presence of the coupled product in the final polymer is not always deleterious it has been difficult to control reproducibly the content of coupled polymer in the carboxy-containing polymers made under the solution polymerization/carbonation process. It is evident that as the coupled polymer content increases the carboxy content of the polymer product will decrease. Thus, if the coupling reaction is difficult to control reproducibly, the carboxy content of the polymer produced under such conditions will also be difficult to control.
It is desirable that the carboxy content of the carboxy-containing polymers employed in thermosetting polyester resin compositions be as high as practicable and that the carboxy content be reproducibly obtained in a process for making such carboxy-containing polymers.